Two Dihydroxo-Bridged Plutonium(IV) Nitrate Dimers and Their

Oct 13, 2015 - The synthesis and structural determination of a dihydroxo-bridged PuIV dimeric moiety, Pu2(OH)2(NO3)6(H2O)4, provide data important to ...
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Two Dihydroxo-Bridged Plutonium(IV) Nitrate Dimers and Their Relevance to Trends in Tetravalent Ion Hydrolysis and Condensation Karah E. Knope,† S. Skanthakumar, and L. Soderholm* Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States S Supporting Information *

ABSTRACT: We report the room temperature synthesis and structural characterization of a μ2-hydroxo-bridged PuIV dimer obtained from an acidic nitric acid solution. The discrete Pu2(OH)2(NO3)6(H2O)4 moiety crystallized with two distinct crystal structures, [Pu2(OH)2(NO3)6(H2O)4]2·11H2O (1) and Pu2(OH)2(NO3)6(H2O)4·2H2O (2), which differ primarily in the number of incorporated water molecules. High-energy X-ray scattering (HEXS) data obtained from the mother liquor showed evidence of a correlation at 3.7(1) Å but only after concentration of the stock solution. This distance is consistent with the dihydroxo-bridged distance of 3.799(1) Å seen in the solid-state structure as well as with the known Pu−Pu distance in PuO2. The structural characterization of a dihydroxo-bridged Pu moiety is discussed in terms of its relevance to the underlying mechanisms of tetravalent metal-ion condensation.

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The condensation of hydrolyzed MIV ions is generally considered to be dominated by olation,3 as evidenced by their hydroxo-bridged metal linkages in solution or in their solid precipitates.6−8 ThIV, defined overall as the softest of the tetravalent ions in terms of the charge-to-radius ratio, exists in HBr solutions as the homoleptic aqua monomer.9 Isolated [Th2(OH)2]6+ dimeric units have been reported in both aqueous solution and solid precipitates.10,11 Similarly, the hardest of the MIV ions stable in aqueous solution, ZrIV and HfIV, also present as μ2OH− oligomers, even under strongly acidic conditions,6,12−15 and yield predominately dihydroxobridged crystalline precipitates.16−18 Their dominant solution species, even under highly acidic conditions, are the dihydroxobridged tetramers [M4(OH)8(H2O16]8+ (M = ZrIV, HfIV).6,14−16 Contrasting with the apparent olation behaviors of ThIV and the transition metals ZrIV and HfIV are those of the tetravalent f ions Ce and Pu, both of which have an ionic radius, and hence a charge-to-radius ratio, intermediate to and bounded by ThIV and HfIV.19 Nitrate solutions have been particularly well studied because of their widespread use in solvent-extraction processes employed in chemical purification and waste reprocessing.20,21 Although CeIV and ThIV have been isolated as isostructural, hexanitrato monomers in the solid state, the PuIV analogue has not.22,23 In addition, there are no published solid-state structural reports for oxo or hydroxo dimeric species for either Ce or Pu. CeIV also has no single-crystal structural reports available for larger clusters except for the hexameric [M6(μ3-

INTRODUCTION Metal cations dissolved in aqueous solution are often present as solvated hydroxo- or oxo-ligated oligomers. This chemistry is understood to have its origin in the Bronsted acidity of the aquated cation, which drives the deprotonation of water:1,2 M(H 2O)n z + + y H2 O ⇌ M(H 2O)n − y (OH)y(z − y) + + y H3 O+

where M is a metal cation, z is the metal charge, and y is the degree of hydrolysis. In general, the magnitude of the first dissociation constant (pKa) for this reaction scales well with the metal-ion’s charge-to-radius ratio, or hardness; the harder the ion, the higher the pKa. Once formed, the hydrolyzed species can condense, a process understood to occur via either olation, to form hydroxo-brigded oligomers, or oxolation, to form oxobridged species, with increasing hardness favoring oxolation. Which condensation pathway is taken depends again on the Bronsted acidity, this time of the hydrolyzed complex,3,4 as well as solution conditions, most notably the metal concentration and pH. The condensation mechanism can be very important because olation tends to result in chemically and structurally illdefined M−OH oligomers and amorphous precipitates, whereas oxolation often produces structurally distinct M−O nanoclusters with well-defined chemistry, as exemplified by the polyoxometallates observed for WVI, MoVI, and NbV.5 We are interested in understanding the details of these condensation mechanisms for MIV cations, with a goal to controlling their chemistry for targeted nanoparticle syntheses and reactivity. © 2015 American Chemical Society

Received: June 3, 2015 Published: October 13, 2015 10192

DOI: 10.1021/acs.inorgchem.5b01242 Inorg. Chem. 2015, 54, 10192−10196

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Inorganic Chemistry O)4(μ3-OH)4]12+ unit, which has been structurally characterized for all of the tetravalent ions M = Zr,17 Ce,24 Th,25−27 U,25 and Pu. 2 8 In contrast, nanoparticles of the large [Pu38O56Cl54(H2O)8]14− cluster have been isolated and their structures determined.29,30 Only Pu−O−Pu linkages occur in this structure, pointing to an oxo-bridged condensation mechanism. There has been no structural report of a dioxobridged Pu species in the solid state, and there is one report of an extended dihydroxo-bridged sulfate,31 a result based on a Zr analogue. In stark contrast to its solid-state structure comprised of isolated, hexanitrato monomers, recent studies of CeIV in perchlorate32 and nitrate33,34 solutions revealed that CeIV presents as a monooxo-bridged dimer. Earlier studies on acidic aqueous plutonium(IV) nitrate solutions found only evidence for the monomeric hexanitrato complex similar to that seen in solid state.35 This is in spite a variety of fundamental studies concerning the formation of soluble PuIV oligomers20,21,36−39 and their contribution to the overall thermodynamic equilibria used to model environmental and separation behaviors.40,41 Studies aimed at determining hydrolyzed PuIV speciation in solution have yielded mixed results. X-ray absorption fine structure (XAFS) Pu−O bond distances have been used to infer hydroxo species,38,42 but the results are inconclusive because the distances are within the range of plutonium oxo bridges found in the Pu-38 cluster,9,29,30 seen in both solution and the crystalline structure. In summary, MIV condensation products reported to date appear to fall into two regimes: (i) Th, Zr, and Hf, which form predominantly hydroxo-bridged condensates; (ii) Ce and Pu, for which purely oxo-bridged species have been conclusively isolated. These findings raise questions about the underlying mechanism(s) driving MIV condensation, particularly with respect to the general discussion of Pu hydrolysis products within an olation regime.21,41−45 The answers to these question are of interest, in part, to enable oxolation over olation, wherein the condensation products are well-defined clusters for which rational surface chemistry can be developed and deployed.46 We describe herein our isolation and single-crystal characterization of dihydroxo-bridged Pu dimers, together with Pu speciation in the mother liquor from which the crystals form.

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corrected for absorption using the APEX2 suite of crystallographic software.47 The structure was solved by direct methods using SHELXS-97 and refined using SHELXL-97 and SHELXL-2014.48 All non-H atoms were found using difference Fourier maps and were ultimately refined anisotropically. H atoms attached to the O atoms of the bridging hydroxide groups (O1) as well as the bound water molecules (O2 and O3) were found during refinement and freely refined for 1. These H atoms were similarly found in the difference Fourier map during the refinement of 2 but were refined with a O−H distance restraint of 0.82(2) Å. H atoms of the solvent water molecules were not located during the refinement of both 1 and 2. Crystallographic details are presented in Table 1.

Table 1. Crystallographic Structure Refinement Details for 1 and 2 1 formula MW (g mol−1) T (K) λ(Mo Kα) μ (mm−1) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z Rint R1 [I > 2σ(I)] wR2 GOF residual density (max/min)

EXPERIMENTAL METHODS

2

[Pu2(OH)2(NO3)6(H2O)4]2· 11H2O 2122.45

Pu2(OH)2(NO3)6(H2O)4· 2H2O 998.17

100 0.7103 orthorhombic Fddd 11.868(1) 18.398(2) 44.979(4) 90 90 90 9821.2(14) 8 0.0266 0.0162

100 0.7103 6.699 monoclinic C2/c 11.9542(7) 16.7688(9) 9.9540(5) 90 92.947(1) 90 1992.72(19) 4 0.0177 0.0105

0.0385 1.078 0.858/−0.810

0.0245 1.091 1.413/−0.491

Powder X-ray diffraction data (Supporting Information) confirm that the crystals used for structure determination are representative of the crystalline component of the sample. Pu Solution Speciation. High-energy X-ray scattering (HEXS) data were collected on selected plutonium nitrate solutions, together with their empty holders and background samples at the Advanced Photon Source (APS), Argonne National Laboratory, on Beamline 11ID-B. The incident-beam energy was set to 86.7 keV, corresponding to a wavelength of 0.143 Å. The experiment was performed in transmission geometry, and the scattered intensity was measured using an amorphous silicon flat-panel X-ray detector mounted in a static position (2θ = 0°) at two different distances, providing detection in a momentum transfer space across Q from 0.2 to 28 Å−1. Data were obtained on room temperature solutions and were treated as described previously.49−51 Spectra were corrected for background scattering (by subtracting an empty sample holder) and polarization and normalized to a cross section per formula unit. Background solutions were used to remove correlations not involving Pu. The reduced, partial SΔ(Q) values were subsequently Fourier-transformed to yield difference-pairdistribution functions (dPDF)s, which only show correlations involving Pu, specifically including those with complexed species, solvent molecules, and nitrate ions. The original HEXS patterns, together with the background-subtracted SΔ(Q) data, are provided in the Supporting Information.

Synthesis of 1 and 2. Caution! 242Pu is an α-emitting radioisotope and requires appropriate inf rastructure and personnel trained in the handling of radioactive materials. The Pu aqueous stock solution used in this study contained 25 mM Pu in approximately 4 M HNO3. It was prepared by initially precipitating Pu from a PuIV/1 M HCl solution with 5 M NaOH. The resulting green precipitate was washed 7−10 times with 5 mL (each wash) of distilled water, after which the precipitate was redissolved in 5 M HNO3. Crystals of compounds [Pu2(OH)2(NO3)6(H2O)4]2·11H2O (1) and Pu2(OH)2(NO3)6(H2O)4·2H2O (2) were prepared nearly quantitatively upon evaporation to dryness of 250 μL of the stock solution (1.5 mg or 6 μmol of 242Pu). This protocol reproducibly yielded green platelike crystals of 1 upon evaporation; however, in one instance, the reaction yielded both green platelike crystals of 1 and green blocks of 2. Optical and Raman spectra of the starting and evaporated solutions, together with those from the solid powder precipitates, are provided in the Supporting Information. Single-Crystal X-ray Structure Determination. Single crystals of 1 and 2 were isolated from the bulk reaction product and mounted on glass fibers in epoxy. Reflections were collected at 100 K on a Bruker AXS Apex II Quazar equipped with an IμS X-ray source (Mo Kα) and Quazar multilayer optics. The data were integrated and 10193

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RESULTS AND DISCUSSION Solid-State Structure. Structures 1 and 2 are both based on the hydroxo-bridged PuIV dimers shown in Figure 1. Each

hexameric motif has been reported for Hf; however, it is enclosed within a much larger (18-mer) oxo−hydroxo sulfate,15,53 a structure also reported for ZrIV54 and suggestive of mixed olation/oxolation products. However, an in situ highenergy X-ray scattering (HEXS) study on the formation of the hexameric unit in aqueous solution has demonstrated a more complex assembly route, involving the condensation of μ2-OH− dimeric solution species through an anion-assisted route.55 Although not providing direct evidence that a mixed olation/ oxolation route for the assembly of the hexameric unit in general, the mechanism of hexamer synthesis does suggest the formation of dihydroxo-bridged dimers in aqueous solution. A similar condensation may pertain for the larger, mixed oxo/ dihydroxo-linked Zr and Hf structures as well as those for Th oligomers.25,26,56 With the exception of the hexameric clusters, no mixed oxo−hydroxo clusters have been reported for PuIV or CeIV. PuIV Solution Speciation. To ascertain the relevance of the dimeric structures 1 and 2 to Pu speciation in the acidic nitric acid solutions from which they precipitated, solute correlations were directly probed using HEXS.57 The dPDFs shown in Figure 2 are obtained by subtracting scattering patterns of

Figure 1. Illustration of the hydroxo-bridged PuIV dimer of 1, the same structure as that seen for 2. Lattice solvent water molecules, which distinguish the two structures, are omitted for clarity. Green, blue, red, and pink spheres are PuIV, N, O, and H atoms, respectively.

PuIV is coordinated to 10 O atoms from three bidentate nitrate anions, two water molecules, and two bridging hydroxide groups. Relevant bond distances are summarized in Table 2. Table 2. Average Pu−O, Pu−N, and Pu−Pu Distances Determined by Analyses of X-ray Diffraction Data Obtained from Single Crystals of 1 and 2 N

distance in 1 (Å)

distance in 2 (Å)

Pu−Oall Pu−Owater Pu−Oμ2‑OH

10 2 2

2.42(10) 2.39(1) 2.249(1)

2.43(10) 2.41(4) 2.25(4)

Pu−Obound nitrate O Pu−Onitrate distal Pu−N Pu···Pu

6 3 3 1

2.49(3) 4.15(1) 2.929(15) 3.799(1)

2.49(3) 4.15(1) 2.932(15) 3.803(1)

Figure 2. dPDF plots of 93 mM Pu in a 5 M HNO3 stock solution (black) compared with the same solution after evaporation to a Pu concentration of 304 mM Pu. Metrical parameters obtained by fits to these patterns are included in Table 3.

The hydroxo-bridged Pu unit described herein and shown in Figure 1 was prepared from aqueous solution under conditions comparable to those used to crystallize the dimeric [Th2(μ2OH)2(NO3)6(H2O)6] moiety.10,11 The observation of a [Pu2(OH)2]6+ nitrate dimer thus bounds the hardness limits for the formation of this dimeric structural motif for the ThIV− PuIV actinide series. Our results suggest that a similar synthetic route may be available for the isolation of similar UIV and NpIV analogues, which have yet to be structurally characterized. In contrast to the 5f actinides, no dimeric solids have been structurally characterized either for CeIV or for the transition metals ZrIV and HfIV. Instead, hydroxo-bridged tetramers [M4(OH)8(H2O)16]8+ (M = ZrIV, HfIV) precipitate from aqueous solution, even under acidic conditions.6,14−16 Extended hydroxo-bridged networks have been observed for a zirconium(IV) sulfate network,52 with one preliminary report of a similar extended Pu network isostructural with the zirconium sulfate compound.31 Larger, mixed oxo−hydroxo-linked MIV aggregates have also been reported to crystallize from acidic aqueous solutions. Of note are the hexanuclear clusters of the form [M6(μ3-O)4(μ3OH)4]12+, which have been structurally characterized for the tetravalent ions of Zr,17 Ce,24 Th,25−27 U,25 and Pu.28 A similar

background solutions from those obtained for metal-containing samples, applying appropriate scaling and form-factor corrections and Fourier-transforming the resulting SΔ(Q).49,50,58 The outcome is a metal-centric pattern with Pu−ligand distances (r) and peak intensities scaled to the number of electrons associated with the correlation. A solution similar to that used to produce crystals containing the dimeric species (93 mM Pu in 5 M HNO3) was used to ensure adequate HEXS statistics. The dPDF from this solution is compared to the same solution after partial evaporation (304 mM Pu; azeotropic in nitric acid) in Figure 2. If left to evaporate, this solution also produces the Pu dimer with almost quantitative yield. The dPDFs from the two solutions appear to be substantially different, reflecting a change in the Pu coordination environment as the solution evaporates. The pattern from the initial stock solution was modeled with Gaussian peaks and assigned ligation based on the structure of the dimer and previously published extended XAFS data on similar solutions.35 The partially resolved peak at 2.88(1) Å is attributed to bidentate10194

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evaporated. Despite the decreased solvent correlations, Pu remains in solution at this concentration. Further evaporation (to 450 mM Pu) does not result in a dPDF significantly different from that seen in Figure 2. There is no evidence for the presence of larger Pu oligomers.

Table 3. Summary of Correlation-Peak Positions, Obtained from the dPDF Patterns Shown in Figure 2a initial solution distance (Å)

evaporated solution distance (Å)

suggested assignment

2.41(1) 2.56(2) 2.88(1)

2.41(1) 2.57(1) 2.87(1)

O+H O Nb (nitrate)

3.70(10)

Nm (nitrate) Pu−Pu

4.12(5)

ObT (nitrate)

3.32(5)

4.12(5) 4.5(1) 4.97(6) a

crystal structure

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2xOH−; 2xOH2 6xO-NO3 3xNb−O3 (bidentate)

CONCLUSIONS The isolation and structural characterization of a dihydroxobridged Pu dimeric unit, Pu2(OH)2(NO3)6(H2O)4, crystallized as 1 and 2, is a result of the importance of understanding the mechanism by which hydrolyzed tetravalent ions condense to form oligomers. ThIV and HfIV, respectively the softest and hardest tetravalent ions stable in aqueous solution, have wellestablished structural examples of the dihydroxo-bridged units resulting from olation condensation. No such examples have existed for either CeIV or PuIV, even though in each case their hardness, defined by the charge-to-radius ratio, is intermediate between that of Th and Hf. Whereas there is no structural report of a small Ce oligomer, the Pu structures reported to date only include the M−O−M linkages indicative of oxolation. The determination of the dimeric nitrate species seen in both 1 and 2 now provide an example that links it to the chemistry seen for Th, Zr, and Hf. HEXS data obtained from concentrated plutonium nitrate solutions similar to those producing the single crystals did show evidence for a Pu−Pu correlation at 3.70(10) Å, although the predominant Pu speciation remained monomeric, even at high concentrations. Unfortunately, whether the dimeric species is dihydroxobridged, as seen with ThIV, ZrIV, and HfIV, or whether it is oxo-bridged, as seen for the larger Pu-38 cluster and for the Ce−O−Ce dimer, could not be distinguished. Although our isolation of the dihydroxo-bridged Pu dimer does link its chemistry to that of Th, Zr, and Hf, the overall picture of Pu hydrolysis chemistry remains more closely linked to that of Ce. Overall, the condensation products seen in solution and in the solid state do not form a well-defined series with increasing hardness, raising the question of what changes are needed in our simple mechanistic rationale of hydrolyzed metal condensation before we have a predictive understanding of this complex chemistry.

Pu−Pu (dihydroxobridged) 3xObT (terminal)

4.65(5) 4.94(6)

The raw data are included as Supporting Information.

coordinating nitrate groups, whereas the broad and weaker peak centered at 3.32(5) Å could indicate an N atom from a monodentate-coordinating nitrate group, but the intensity indicates its presence as only a minor fraction of the ligating NO3−. There are no other correlations present, even in the background solution, that could account for this peak. Monodentate-coordinated nitrate ligation is not observed in the dimer, nor has it been reported in other plutonium nitrate structures. The broad region of correlation peaks at higher r is expected for a combination of the terminal O atom from the bidentate nitrate and outer-sphere complexed-solvent molecules on a rising background of disordered solution correlations.58 There is no evidence in the dPDF of a correlation peak near 3.75−3.85 Å, the Pu−Pu distance seen in the solid-state hydroxo-bridged dimers, an adjacent Pu in the μ3-hydroxo/oxo-bridged hexamer,28 or a Pu−O−Pu linkage as found in PuO2. The absence of such dimers supports general thermodynamic models that do not include this species in Pu solutions at infinite dilution.59 Modeling the dPDF of the evaporated solution with a Pu concentration of 304 mM in a similar fashion reveals three significant changes in the Pu coordination environment that accompany the increased plutonium nitrate concentration. First, although the three Gaussians used to model the first coordination sphere O/H and bidentate N coordination do not shift, the intensity of the unresolved peak at about 2.56(3) Å increases markedly, giving the perception that peaks at 2.41 and 2.87 Å have shifted, although they have not. The increase in the 2.56(3) Å peak intensity suggests an increase in the number of coordinating nitrates, a result consistent with the solution chemistry. Second, the notable difference between the patterns is the disappearance of the peak at 3.35 Å, replaced by a weak and broad peak centered at about 3.7 Å. This distance is similar to the Pu−Pu distance in a dihydroxo-bridged Pu interaction, as seen in 1 and 2, and its origin is therefore consistent with the presence a small amount of the dimer, relative to the monomeric species. However, it should be noted that the distance is also consistent with the oxo linkage Pu−O−Pu seen in PuO2. There are no other correlations present, even in the background solution, that could account for the observed peak at 3.7 Å. The intensity of the peak represents a maximum concentration of the Pu dimer at less than 25% of the total Pu speciation. Third is the marked decrease of the scattering centered at about 4.5 Å. This change arises from a decrease in the outer-sphere solvent correlations because the solution has

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01242. Powder X-ray diffraction pattern of the precipitate resulting from the synthesis of 1, X-ray structure determination information, ORTEP illustrations and structural details for compounds 1 and 2, HEXS patterns for a stock solution and background, together with SΔ(Q) data for the solutions before and after evaporation, and optical and Raman data for solutions before and after evaporation and for solid precipitate 1 (PDF) Crystallographic data, which have also been deposited with the Inorganic Crystal Structure Database and may be obtained at http://icsd.fiz-karlsruhe.de (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 10195

DOI: 10.1021/acs.inorgchem.5b01242 Inorg. Chem. 2015, 54, 10192−10196

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†

Department of Chemistry, Georgetown University, 37th and O Streets NW, Washington, DC 20057. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported at University of Chicago at Argonne, LLC (Argonne National Laboratory), by the U.S. Department of Energy, BES Heavy Elements Program, under Contract DEAC02-06CH11357. The APS, a U.S. DOE Office of Science User Facility, is operated at Argonne National Laboratory under the same contract number.

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DOI: 10.1021/acs.inorgchem.5b01242 Inorg. Chem. 2015, 54, 10192−10196